A defining characteristic of a compound is that it has a chemical formula. Formulas describe the ratio of atoms in a substance, and the number of atoms in a single molecule of the substance (thus the formula for ethene is C2H4 rather than CH2). The formula does not indicate that a compound is composed of molecules; for example, sodium chloride (table salt, NaCl is an ionic compound.

Compounds may have a number of possible phases of matter [1]. All compounds can exist as solids. Molecular compounds may also exist as liquids or gases. All compounds will decompose to smaller compounds or individual atoms if heated to a certain temperature (called the decomposition temperature [2]). Every chemical compound that has been described in the literature carries a unique numerical [3] identifier [4], its CAS number [5].